Images produced using dye-based inks from an inkjet printer have achieved photographic quality results in recent years. However, dye-based images are known to suffer from limitations in image stability from a durability and image permanence (light and ozone fade) standpoint. Images produced using pigment-based inks are generally much better for image permanence than dye-based inks; however, images formed from pigment-based inks are more challenged from a durability and print uniformity standpoint than dye-based inks.
Print uniformity issues of particular concern are gloss-banding and mottle. Bidirectional printing of pigment ink images often results in a non-uniform image which has the appearance of bands of alternating gloss when the image is viewed at near the specular angle. This phenomenon is herein referred to as gloss-banding and is detrimental to the overall quality of the image. Another aspect of print uniformity is uniform density or absence of visual print mottle. That is, a target printed with a uniform given nominal density should be free of visually observable variations in print density. A metric for visible variations in print density is the standard deviation of a set of L* values of about 400 micron square areas within the target. The smaller the standard deviation the smoother or more uniform is the apparent print density.
When jetted onto porous glossy inkjet recording media, pigment ink components, including the pigment dispersions and attendant polymers, are filtered at the surface of the media. The gloss-banding image artifact derives from differences in light scatter off the newly created surface formed by the jetted ink. There are many asymmetries in overlap printing or bidirectional printing that can give rise to surfaces that scatter light differently. One such asymmetry is the number of nozzles used within a given raster line. An approach to suppressing gloss-banding that has been disclosed (US Publication No. 2007/0008364) is to select a print mode where the number of dots per unit area is small, as noted however this causes another artifact, density variation banding. A print mode specifies the amount of each color ink to print to reproduce the desired color, as well as the number of passes of the printhead over the recording medium in which to deposit the ink, and the amount and pattern of each ink to be printed on each pass of the printhead. Other print mode solutions to the problem which may address the asymmetries include printing in only one direction, or increasing the number of passes required to deposit the ink, or by changing the order of printing of the colorants. Each of these approaches will decrease productivity, or change the ink flux which can contribute to other artifacts, such as mottle. In addition, a print mode which is successful in producing a uniform surface on one recording medium may not be successful on another recording medium. Ideally, a method of gloss-band suppression which is not dependent on the particular print mode would be preferred.
As noted, there are many asymmetries arising from bidirectional printing that can result in different surface structures and consequent light scattering properties in the bands. One particular asymmetry is the age of the surface created by previously deposited drops as the head traverses along subsequent print rasters. The appearance of bands of alternating gloss (hazy light scattering bands alternating with glossy light reflective bands) are particularly strong along the edges of the image. Further, within each band the polarity changes from glossy to hazy across the raster. This suggests that the time scale responsible for the phenomena is on the order of a head traverse, and thus consistent with the known kinetics of pH dependent carboxylate containing polymer conformational transitions. These transitions convert hydrophilic polymers into hydrophobic polymers reducing the wettability and permeability of the filtered pigment polymer image layer. Carboxylate containing polymers are none the less very useful ink adjuvants that can serve many functions in an inkjet ink, including serving as binders for the pigment image and jetting aids. The composition of these polymers must balance the contrasting requirements for aqueous dispersibility in the alkaline ink with the requirement for water-fastness of the pigment image on the media surface. Thus, copolymers comprising a carboxylate monomer for aqueous dispersibility, and a hydrophobic monomer for water-fastness are generally employed. Accordingly, it would be very useful to have ink compositions that contain print uniformity improving polymers that, by resisting the aforementioned transitions, are capable of suppressing gloss-banding artifacts.
Print uniformity is also a function of the evenness of printed ink density across the surface of the image. In an ideal case, each ink droplet is jetted to the image recording medium such that the drops penetrate the permeable medium surface, spread across the medium, and joined together to create a uniform image density. In bi-directional printing this is rarely the case. In reality the random placement of droplets, permeability of drops placed onto previously fired drops, inhomogeneity of the recording medium surface, surface tensions of the inks, presence of pigment particles in the ink, and capacity of the medium to absorb liquid all can affect the evenness of the printed ink density across the image. Each of these factors can contribute to an uneven density across the image that has the appearance of mottle to the observer. In some cases the mottle can be ascribed to density variations on the order of about 400 microns on the recording medium surface. Undesirable conditions can exist between the ink and recording medium whereby a significant number of ink droplets merge or coalesce prior to penetration into the image recording medium surface. This droplet coalescence can be a particular problem for pigmented inks since the printed pigment particles reside at or near the surface of the image recording medium thereby amplifying the unevenness in printed ink density. The coalescence of ink droplets on the recording medium can result in a mottled appearance which is highly objectionable to the observer. The present inventors have discovered that the presence of certain print uniformity improving polymers in pigmented inks can greatly minimize the mottle and improve the overall evenness of the printed ink density.
It would be desirable to have a pigment-based ink that can be printed with reduced gloss-banding and mottle. It is further desirable to provide a pigment ink that can be printed using an inkjet printer to a medium having a wide range of surface pH without gloss-banding or mottle. A yet further desire, is a pigment-based ink that does not gloss band or show mottle over a range of print modes.